Method and apparatus for improving the comfort of CPAP

ABSTRACT

A low-cost CPAP apparatus in which, upon detection of the transition from inhalation to exhalation, the blower motor is de-energized to allow it to freewheel. When the pressure in the patient mask (or whatever interface is utilized) reaches a minimum pressure level during exhalation, the motor is re-energized and its speed is controlled so to maintain the pressure at a level suitable for exhalation. Upon detection of the transition from exhalation to inhalation, the motor speed is increased to provide higher pressures in the patient mask suitable for inhalation.

This application claims the foreign priority of Australian provisionalapplication No. AU 2003903138 filed on Jun. 20, 2003.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for delivering a supplyof air at positive pressure to a patient for treating sleep disorderedbreathing. In particular, the invention relates to a method andapparatus which provides a smooth, comfortable pressure versus timewaveform.

BACKGROUND OF THE INVENTION

Since the invention of nasal Continuous Positive Airway Pressure (nasalCPAP) for treatment of Obstructive Sleep Apnea (OSA) and other forms ofSleep Disordered Breathing (SDB) by Sullivan, as taught in U.S. Pat. No.4,944,310, much effort has been directed towards improving the comfortof the devices. One aspect of this is a more comfortable patientinterface, such as provided by the MIRAGE® and ULTRA MIRAGE® masksmanufactured by ResMed Limited. Another aspect of providing a morecomfortable patient interface is the comfort of the waveform of air atpositive pressure provided by the blower.

Some low cost CPAP blower devices, such as the S7™ device by ResMedLimited, provide a supply of air at a generally fixed positive pressurethroughout the respiratory cycle of the patient, for example, 15 cmH2O.A blower comprising an electric motor and fan can be constructed todeliver air based on a rotational speed of the motor predetermined todeliver a particular pressure to a patient interface, such as a mask.When the patient breathes in with such a system, the pressure in themask may reduce by a small amount. When the patient breathes out withsuch a system, the pressure in the mask may increase by a small amount.These fluctuations in mask pressure are referred to as “swing”. Otherblowers use feedback in a pressure controller which counterbalances theeffect of patient effort on the mask pressure to reduces the swing. Sucha device has a current retail price in Australia of approximately AU$1000.

Another group of CPAP devices, such as the ResMed AUTOSET® SPIRIT™device can monitor the patient and determine an appropriate CPAP settingto deliver to the patient, which pressure may vary through the night,for example, delivering 15 cmH2O during an initial portion of thepatient's sleep, but increasing to 20 cmH2O later in the night. Changesin pressure are made in response to a determination of the occurrenceand severity of aspects of breathing such as flow limitation andsnoring. Such a device has a current retail price in Australia ofapproximately AU $2000.

A bi-level CPAP device, such as the ResMed VPAP® product, provides ahigher pressure to the patient's mask during the inspiratory portion ofthe respiratory cycle, for example, 10-20 cmH2O, and a lower pressureduring the expiratory portion of the patient's breathing cycle, forexample, 4-10 cmH2O. A mismatch between the device control cycle and thepatient respiratory cycle can lead to patient discomfort. When thedevice makes a transition from the higher pressure to the lower pressurethe motor is braked. When the device makes the transition from the lowerpressure to the higher pressure, the motor is accelerated. Depending ondevice settings, the device may be required to make a change of 5-18cmH2O pressure in 50-100 ms. To achieve this change, the peak power loadmay be in the order of 60-90 W. Because of the low inertia and peak loadrequirements of the VPAP® device, a large and expensive power supply isrequired. Such a device has a current retail price in Australia ofapproximately AU $3,500-7,500 depending on the device feature set.

U.S. Pat. No. 6,345,619 (Finn) describes a CPAP device that provides airat a pressure intermediate the IPAP (Inspiratory Positive AirwayPressure) and EPAP (Expiratory Positive airway Pressure) pressuresduring the transition between the inspiratory and expiratory portions ofthe device control cycle. U.S. Pat. Nos. 6,484,719 (Berthon-Jones) and6,532,957 (Berthon-Jones) describe devices which provide pressuresupport in accordance with a waveform template. U.S. Pat. No. 6,553,992(Berthon-Jones et al.) describes a ventilator whose servo-controlleradjusts the degree of support by adjusting the profile of the pressurewaveform as well as the pressure modulation amplitude. As theservo-controller increases the degree of support by increasing thepressure modulation amplitude, it also generates a progressively moresquare, and therefore efficient, pressure waveform; when theservo-controller decreases the degree of support by decreasing thepressure modulation amplitude, it also generates a progressively moresmooth and therefore comfortable pressure waveform. The contents of allof these patents are hereby incorporated by reference.

CPAP and VPAP devices are mechanical ventilators. Ventilators have beenclassified (Chatburn, Principles and Practice of Mechanical Ventilation,Edited by M J Tobin, McGraw Hill, 1994, Ch. 2) as being either pressure,volume or flow controllers. In each case, the ventilator controls thepressure of air versus time, volume of air versus time, or flow of airversus time that is delivered to the patient. Many such devices can beprogrammed to deliver a variety of waveforms, such as pulse(rectangular), exponential, ramp and sinusoidal. The shape of thewaveform actually delivered to the patient may be affected by thecompliance and resistance of the patient's respiratory system and hisbreathing effort, as well as mechanical constraints such as blowermomentum and propagation delays.

The Siemens Servo Ventilator 900B is a pneumatically powered ventilatorwhich uses a scissor-like valve to control the inspiratory flow pattern(McPherson & Spearman, Respiratory Therapy Equipment, The C. V. MosbyCompany, 1985, pp. 469-474).

Ventilators have been constructed to deliver an inspiratory waveformwhen one of pressure, volume, flow or time reaches a preset value. Thevariable of interest is considered an initiating or trigger variable.Time and pressure triggers are common. The Puritan Bennett 7200aventilator is flow triggered. The Dräger Babylog ventilator is volumetriggered. The Infrasonics Star Sync module allows triggering of theInfant Star ventilator by chest wall movement. The ventilator'sinspiration cycle ends because some variable has reached a preset value.The variable that is measured and used to terminate inspiration iscalled the cycle variable. Time and volume cycled ventilators are known.

Many ventilators provide a Positive End-Expiratory Pressure (PEEP). Someof these ventilators use a valve (the PEEP valve) which allows the PEEPto be varied. Some devices, such as that taught by Ernst et al. in U.S.Pat. No. 3,961,627, provide a combination of pressure and flow controlwithin one respiration cycle. A control cycle is divided into fourphases I, II, III and IV. The respiration cycle and the control cycle donot necessarily have to fall together in time; mostly, however, phases Iand II of the control cycle correspond to inspiration, and phases IIIand IV of the control cycle correspond to expiration. Phases I, III andIV are pressure-regulated, and phase II is flow-regulated. The doctorcan choose the pressure course with the three control elements for theexpiratory pressure decrease, the inflexion, and the final expiratorypressure. In phase III, the pressure proceeds from the pressure measuredat the end of phase II according to a fixed pressure decrease dP/dt.When the pressure measured in phase III reaches the inflexion, thepressure proceeds linearly to the fixed final expiratory pressure. Thepart of the expiration from the inflexion to the end of the respirationcycle represents phase IV. The linear course of the pressure in theexpiration represents a preferred embodiment, but could be replaced byanother course of the pressure curve, for example, an exponential.

A spontaneously breathing patient exerts at least some effort to breath,however inadequate. A lack of synchrony between the respiratory cycle ofthe patient and that of the ventilator can lead to patient discomfort.

In Proportional Assist Ventilation (PAV), as described by Magdy Younes,the ventilator generates pressure in proportion to patient effort; themore the patient pulls, the higher the pressure generated by themachine. The ventilator simply amplifies patient effort without imposingany ventilatory or pressure targets. It is understood that the objectiveof PAV is to allow the patient to comfortably attain whateverventilation and breathing pattern his or her control system sees fit.The PAV system is further discussed in U.S. Pat. Nos. 5,044,362,5,107,830, 5,540,222 and 5,884,662.

In U.S. Pat. No. 5,044,362, Younes describes a system that operates asfollows:

(a) Inspired flow: When the high frequency components of the output of avelocity transducer in a line are filtered out, the remaining signalagrees very well with flow measured independently at the airway.Accordingly, the velocity signal in the line is passed through a lowpass filter and the resulting signal is used as a command signal for theventilator unit to produce pressure in proportion to inspired flow,which is said to provide resistive assist. A gain control permits theselection of the magnitude of the assist. In practice, the flow signalis permanently connected to a summing amplifier and a minimum gain isset to offset the resistance of the tubing. When a greater assist isrequired to offset the patient's own resistance, the gain is increased.

(b) Inspired volume: The signal related to inspired flow may beintegrated to provide a measure of inspired volume. A signalproportional to pressure is subtracted to allow for piston chambercompression. The magnitude of the pressure signal that is subtracted isa function of the gas volume of the system, according to Boyle's law.When the resulting signal is routed to the summing amplifier, theventilator unit develops pressure in proportion to inspired volume. Themagnitude of the assist obtained again may be controlled by a gaindevice.

(c) Ramp generator: This mode of operation permits the ventilator unitto function independent of patient effort and provides a controlledventilation. This function can be activated by the operator throwing aswitch to bring the function generator into the circuit. Alternatively,provision may be made for the ramp generator to be routed automaticallyto the summing amplifier in the event of the failure of the patient tobreathe spontaneously for a specified period of time.

(d) D.C. output: An adjustable DC output provided by an offset amplifieralso is routed to the summing amplifier, to result in the generation ofcontinuous pressure.

U.S. Pat. No. 5,535,738 (Estes et al.) describes a further PAVapparatus.

Life support ventilators such as the Puritan-Bennett 7200a allow for avery precise control of the pressure, volume and flow rates of airdelivered to patients. However such life support ventilators are tooexpensive for home use in treating sleep disordered breathing costing inthe order of AU $50,000 to 100,000.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided alow-cost CPAP apparatus that provides a more comfortable waveform of airfor treatment of sleep disordered breathing.

In accordance with another aspect of the invention there is provided alow-cost CPAP apparatus for treatment of sleep disordered breathing thatprovides a reduced pressure of air during exhalation.

In accordance with another aspect of the invention there is provided alow-cost CPAP apparatus for treatment of sleep disordered breathing thatreduces pressure during exhalation from a first level to a second level,the second level being no lower than a predetermined limit.

In accordance with another aspect of the invention there is provided amethod of controlling the pressure developed by a blower by an electricmotor comprising the step of freewheeling the motor when the pressuredeveloped by the blower exceeds a threshold.

In accordance with another aspect of the invention, there is provided aCPAP apparatus which provides air at a first positive pressure during aninspiratory portion of a control cycle of the apparatus and air at asecond positive pressure during an expiratory portion of a control cycleof the apparatus, and a smoothly varying pressure waveform between theinspiratory and expiratory portions of the control cycle.

In accordance with another aspect of the invention, there is provided aCPAP apparatus comprising an electric motor, a motor speed controller,an impeller (as used herein, an impeller is a rotating mechanism drivenby a motor that blows air), a volute, a flow sensor, a controller, anair delivery conduit and a patient interface wherein:

(i) the CPAP apparatus is adapted to provide air at a first positivepressure in the patient interface during an inspiratory portion of acontrol cycle;

(ii) the flow sensor is adapted to measure the flow rate of air throughthe air delivery conduit;

(iii) the pressure sensor is adapted to measure the pressure of air inthe patient interface;

(iv) the controller is adapted to detect a signal from the flow sensorindicative of the beginning of an expiratory portion of the respiratorycycle of the patient;

(v) upon detection of the beginning of the expiratory portion of therespiratory cycle, the controller is adapted to instruct the motor speedcontroller to freewheel the motor until the pressure in the patientinterface falls to a predetermined level, and thereafter the controllerinstructs the motor controller to control the speed of the electricmotor to deliver air at a second positive pressure; and

(vi) upon detection of either an apnea or the beginning of theinspiratory portion of the patient's respiratory cycle, the controlleris adapted to instruct the motor speed controller to accelerate themotor until the pressure in the patient interface reaches the firstpositive pressure in the patient interface.

Another aspect of the invention is to provide a CPAP apparatus with alow inertia motor. In accordance with this aspect, the motor inertia issufficiently low so that the motor can freewheel from a first speed thatdelivered a first pressure level until a second predetermined pressurelevel is reached prior to the end of the expiratory cycle. In accordancewith this aspect, the inertia of the motor is sufficiently low so thatthe rate of deceleration to the second predetermined pressure level isfast relative to the typical duration of exhalation. Preferably, theinertia of the motor and/or impeller is less than 13,600 g.mm². The netresult is that a low-cost CPAP apparatus for treatment of sleepdisordered breathing can be manufactured by having a blower driven by amotor provide air at a desired pressure at the airway of a patient inwhich the pressure during exhalation is reduced from a high level to alow level by allowing the motor to freewheel.

In another aspect of the invention, the first pressure level isdetermined in accordance with an automatic algorithm such as thatdescribed in U.S. No. Pat. 5,704,345.

Another aspect of the invention is to provide CPAP apparatus with amotor capable of delivering a more comfortable waveform of air.

Another aspect of the invention is to provide a CPAP comfort mode with amaximum pressure drop.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows CPAP apparatus in accordance with an embodiment of theinvention;

FIG. 2 shows the response of apparatus in accordance with an embodimentof the invention that uses freewheeling to slow the motor;

FIG. 3 shows the response of apparatus in accordance with an embodimentof the invention that uses a combination of braking and freewheeling;

FIG. 4 shows the response of apparatus in accordance with an embodimentof the invention that uses braking with a 33% duty cycle;

FIG. 5 shows the response of apparatus in accordance with an embodimentof the invention that uses negative peak detection to determine when toincrease the pressure;

FIGS. 6 a-d show various views of a lower volute suitable for use withan apparatus in accordance with an embodiment of the invention;

FIGS. 7 a-f show various views of an upper volute suitable for use withan apparatus in accordance with an embodiment of the invention;

FIGS. 8 a-d shows further views of an upper volute suitable for use withan apparatus in accordance with an embodiment of the invention;

FIGS. 9 a-c shows various views of a motor and impeller assemblysuitable for use in apparatus in accordance with an embodiment of theinvention;

FIG. 10 shows the response of an apparatus in accordance with anembodiment of the invention with CPAP pressure set to 15 cm H₂O and alevel of reduction of 3 cm H₂O, the upper curve showing flow, the lowercurve showing pressure, and each horizontal grid corresponding toapproximately 1 second; and

FIG. 11, with the same horizontal and vertical scales as FIG. 10, showsa view similar to that of FIG. 10, but with the CPAP pressure set to 10cm H₂O.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows apparatus in accordance with an embodiment of ourinvention. A brushless electric motor 10 has an impeller 12 attached toit. The impeller 12 resides in a volute 36. The motor 10 is under thecontrol of a motor controller 24 (suitable controllers includeTMS320LC2402 or MC33035 IC). The motor includes sensors 32, 34 thatprovide signals indicative of motor rotational speed and currentrespectively. When the windings of the motor are energized, the impellerrotates. Air is drawn in through the inlet of the impeller and gainsmomentum. As the air passes out of the impeller and into the volute, itchanges speed and develops pressure. Air passes out of the volute, pastflow and pressure sensors 28, 30 (such as SM15652-003 flow sensor andSMI5652-008 or MPX2010 pressure sensors) respectively to an air deliveryconduit 16 (for example, manufactured by Smooth-bor Plastics) that is inturn connected to a patient interface 18 which in the illustratedembodiment is a nasal mask, for example, a MIRAGE® or ULTRA MIRAGE® maskmanufactured by ResMed Limited. Other forms of patient interface may beused, for example, a full-face mask, nasal prongs and nasal cushions.

The flow and pressure sensors 28, 30 provide data to a microcontroller14. Suitable microcontrollers include the HITACHI SH 7032/34 which are32-bit RISC devices, with a clock rate of 2-20MHz, 8 by 10 bit A-Dconverters and a variety of Input and Output features. Themicrocontroller 14 uses the Nucleus Plus Real-time Operating System(RTOS) by Accelerated Technologies Incorporated. In one form of theinvention the apparatus delivers a predetermined CPAP pressure; such adevice is the S7 ELITE by ResMed Limited. In another form of theinvention the microcontroller 14 is programmed to deliver CPAP therapyin accordance with U.S. Pat. No. 5,704,345 (Berthon-Jones) which teachesa method and apparatus for detection of apnea and obstruction of theairway in the respiratory system. The contents of U.S. Pat. No.5,704,345 are hereby incorporated by cross-reference.

The microcontroller 14 is programmed to detect the patient's transitionbetween the inspiratory and expiratory portions of the patient'srespiratory cycle. There are a number of ways of accomplishing thisdetection. One way is to monitor the flow of air to and from thepatient. When the respiratory flow signal crosses zero (a“zero-crossing”) there has been a transition between inspiration andexpiration. Alternatively or additionally, the mask pressure may bemonitored. When mask pressure falls below a first pressure threshold,inhalation is taken to have occurred. When mask pressure rises above asecond threshold, exhalation is taken to have occurred. Alternatively oradditionally, an effort sensor may be used on the patient, for example,to determine chest wall movements, movement of the supra-sternal notchor other patient movements (for example as described in U.S. Pat. No.6,445,942). Measurement techniques may be combined with timed estimatesof the period of a breath. For example, an average breath rate of thepatient may be measured and when inhalation is detected by a flow,pressure or effort sensor, exhalation is assumed to occur after a periodbased on the measured average breath rate.

The apparatus includes a display 22, for example, a 2 line by 16character LCD or similar display device. The apparatus includes a keypad26, such as one using backlit silicone switches. The device alsoincludes a power supply which provides 40W at 24V with Class IIisolation manufactured by SKYNET. The apparatus may include an interface20 to enable communication with external devices. For example, asuitable interface chip is the MAX3130/MAX3131 from MAXIM. These chipsprovide both IrDA and RS-232 communication.

Inertia is the tendency of matter to remain at rest, if at rest, or, ifmoving, to keep moving in the same direction unless acted upon by someoutside force. A motor and impeller suitable for use with the inventionshould be of sufficiently low inertia. In use for a CPAP device, themotor typically operates at several thousand revolutions per minute(RPM). If power is no longer supplied to a moving motor, the time ittakes to slow down is a function of the inertia of the motor andimpeller and the air flow rate through the impeller. A given motor,impeller and volute combination will develop lower pressure air at alower rotational speed. In a preferred form of the invention the inertiais less than 13,000 to 14,000 g. mm², preferably less than 13,600 g.mm². One form of suitable volute housing is shown in FIGS. 6-8, and oneform of suitable motor and impeller is shown in FIG. 9. A suitablematerial for constructing the impeller is a glass filled polycarbonatesuch as GE LEXAN 3412R. Such a volute and impeller are found in the S7™ELITE by ResMed Limited.

Inertia, or moment of inertia, is the term given to rotational inertia,the rotational analog of mass for linear motion. Moment of inertia isdefined with respect to a specific rotation axis, in our case, the axisof the spindle in the motor. The moment of inertia of a point mass withrespect to this axis is defined as the product of the mass times thedistance from the axis squared. The moment of inertia of any extendedobject is built up from that basic definition.

Inertia for a complex article may be calculated using a computer packagesuch as SolidWorks. The package takes the rotor, spindle, impeller andanything else that rotates, breaks it all into millions of blocks ofmass, and adds all the individual Inertias together (J. L. Meriam and L.G. Kraige, Engineering Mechanics—Dynamics, John Wiley & Sons, Inc.,1998, pages 665-682).

Alternatively, one may calculate the moment of inertia from firstprinciples. The rotor may be broken into simple shapes such as discs,cylinders, spheres etc. For these shapes, the inertia values can becalculated analytically and then added together to get the total momentof inertia.

A CPAP device such as the S7 ELITE manufactured by ResMed Limited may beadapted to incorporate our invention. Generally, the controller of theCPAP apparatus according to an embodiment of our invention is programmedto deliver a CPAP pressure in the range of 4 to 25 cm H2O. In anautomatically adjusting form of the apparatus, such as the AutoSetSPIRIT and as taught in the abovementioned U.S. Pat. No. 5,704,345, theCPAP apparatus is programmed to increase the CPAP pressure to overcomeor prevent partial or complete obstructions of the airway as indicatedby the presence of snoring, apneas or flow flattening.

The controller 14 is programmed to slightly reduce the pressure of airin the patient interface delivered to the patient during the exhalationportion of the respiratory cycle of the patient from the treatmentpressure recommended by the patient's physician. The amount of thereduction is, in one form, determined by a setting accessible throughthe switches 26 on the CPAP apparatus. For example, the controller maybe programmed to provide four different levels of pressure reductionduring exhalation, each level leading to a reduction by an additional 1cm H2O pressure (comfort mode). For example, when the CPAP pressure isset to deliver air at 15 cm H2O pressure to the patient interface duringinhalation and the comfort mode is set to level 2, the pressure will bereduced to 13 cm H2O during exhalation.

In another form of our invention, the amounts of reduction areapproximately 2.5, 3, and 3.5 cm H2O; in further forms of our invention,the amounts of reduction are approximately 1, 2 and 3.5 cm H2O, and 1, 2and 3 cm H2O.

In one preferred form of our invention, the maximum amount of pressurereduction is fixed by the level setting, and during exhalation thepressure Will not drop lower than the predetermined amount. Preferably,the mask pressure will not drop lower than 4 cm H2O.

In one form of our invention, the pressure is reduced from the CPAPpressure by the amount set according to the level setting when a flowsignal monitoring the respiratory flow is indicative of the beginning ofthe expiratory portion of the patient's respiratory cycle. The pressureremains at the lower level until an indication of the beginning ofinspiration is detected, whereupon the pressure is returned to the CPAPpressure.

In another form of our invention, the pressure is reduced from the CPAPpressure by the amount set according to the level setting when a flowsignal monitoring the respiratory flow is indicative of the beginning ofthe expiratory portion of the patient's respiratory cycle. The pressureremains at the lower level until either an indication of the beginningof inspiration is detected, or an apnea is detected, whereupon thepressure is returned to the CPAP pressure.

Upon detection of an apnea, the pressure may remain at the higher levelduring both inhalation and exhalation. In addition, upon the detectionof cessation of the apnea (i.e., resumption of normal breathing) orexpiration of a preset period of time, the reduction of pressure duringexhalation mode may be resumed.

In accordance with a feature of our invention, the pressure reduction iseffected by “freewheeling” the motor. That is, when exhalation isdetected, energization of the motor windings is temporarily stopped,allowing the rotor to slow down eventually and resulting in a reducedpressure of air being delivered to the patient interface. This approachprovides a gradual reduction in pressure that is particularlycomfortable. Pressure in the patient interface is monitored and when thepatient interface pressure drops to an acceptable level (as set by the“comfort” level), energization of the motor windings is resumed at alower speed which is sufficient to maintain the patient interfacepressure at a lower level. If a flow signal indicative of the beginningof inspiration is detected, the motor speed is increased so that itdelivers the required CPAP pressure.

In general, the sequencing takes three steps:

(i) Upon detection of the transition from inhalation to exhalation, themotor is de-energized to allow it to freewheel.

(ii) When the pressure in the patient mask (or whatever interface isutilized) reaches a minimum pressure level during exhalation, the motoris re-energized and its speed is controlled so to maintain the pressureat a level suitable for exhalation.

(iii) Upon detection of the transition from exhalation to inhalation,the motor speed is increased to provide higher pressures in the patientmask suitable for inhalation.

FIG. 2 is a screen from an oscilloscope showing two traces whenapparatus in accordance with an embodiment of the invention is testedwith a breathing machine delivering a sinusoidal breath with a tidalvolume of 500 ml and 15 breaths per minute. The upper trace shows apressure signal. The lower trace shows a flow signal. The flow signal isused to detect at least one transition. The transition of the flowsignal from a positive value to a negative value (a zero crossing) canbe used to define the onset of exhalation. The transition of the flowsignal from a negative value to a positive value (a zero crossing) canbe used to define the onset of inhalation. When the flow signal issmaller than a threshold value, a zero-crossing is taken to haveoccurred. The threshold has the effect of adjusting the phase delaybetween the pressure and flow curves. The larger the threshold, the moreconfidence there can be that a zero-crossing has been detected reliably.However, there is increased uncertainty in when the transition occurred,potentially leading to a phase delay and reduced synchronicity. Definingan inspiratory flow to be a positive value, the threshold for thetransition from inhalation to exhalation is 0 to 5 l/min, preferably 1l/min. The threshold for the transition from exhalation to inhalation is−4 to −1 l/min, preferably −1 l/min.

In another form of the invention, the threshold for the transition frominhalation to exhalation is in the range of −4 to −1 l/min, preferably,−1 l/min, and the threshold from exhalation to inhalation is in therange of 0 to 5 l/min, preferably 1 l/min. An advantage of this approachis that if there is a plateau region in the flow curve from exhalationto inhalation, as sometimes occurs, inhalation will not be triggeredprematurely.

In a further form of the invention, flow is integrated to calculate avolume. Exhalation is taken to occur when the volume is negative, andinhalation is taken to occur when the volume is positive.

In general, in a device in accordance with the invention, specificity ofthe cycling/triggering threshold is favored over sensitivity.

The results of “freewheeling” indicate a long ramping down ofapproximately 3 to 4 cm H2O in approximately 1.7 seconds to the new setpressure during exhalation, and a fast rise time (approximately 350 to800 ms) once inhalation begins. Rise time is a function of a number offactors including power supply and inertia. The long fall time isattributed to the inertia of the motor and no active breaking.

In other forms of our invention, pressure reduction is influenced bybraking the motor during at least some of the time that it isfreewheeling. Braking may be accomplished by shorting the windings. Inone form of the invention, both pressure and flow are monitored byrespective pressure and flow transducers. In other forms of theinvention, pressure is estimated from the motor speed.

FIG. 3 shows the same set of curves as FIG. 2. However, in FIG. 3,following the zero crossing from inhalation to exhalation, a brake isapplied causing the pressure to fall approximately 2.7 cm H2O in 240 ms.The braking times used are a function of the error, that is, thedifference between the measured pressure and the target pressure. Thegreater the error, the longer the duration of braking. As the errordecreases, shorter braking is used. Thus the duration of the brakingtime is a function of the error. In FIG. 2, braking durations of 50 mson/off, 30 ms on/off and 10 ms on/off were used. Hence the pressurereduction occurs as a result of a combination of freewheeling andbraking of the motor.

In one form, the brake is applied and not released until the pressure onthe transducer is the same as the new set pressure for the exhalationcycle. If the brake is on when inhalation begins (i.e., RespiratoryFlow, R_(f)>H_(TH)), then the brake is released and the set pressurereturned to the CPAP treatment pressure.

A state machine was used to apply the brake in on/off cycles with theperiod getting faster as the set pressure is reached.

The application of a switching brake can be further improved by takinginto account braking over large and small pressure drops.

An algorithm for accomplishing the pressure reduction is as follows:

1) Detect zero crossing.

2) Set Target pressure=(Treatment pressure−Selected pressure reduction).

3) Calculate pressure error=Current pressure−Target pressure

4) While pressure error>=a threshold, brake the motor with a preferredduty cycle.

In one form, dithered braking is used, that is, braking the motor inshort bursts, for example a 25%-50%, preferably 33%, duty cycle (e.g.,10 ms brake on, 20 ms brake off). The size of the duty cycle required toachieve the required pressure reduction is a function of inertia. With amotor and impeller combination as described above, use of a shorter dutycycle—say 10%—is insufficient to slow the motor down to achieve thedesired pressure reduction in the desired time. However, with the use ofa lower inertia motor and impeller, it may be possible to achieve thedesired pressure reduction with a 10% duty cycle. Furthermore, whilewith the preferred motor and impeller use of a duty cycle greater thanabout 50% can cause pressure spikes, when other motors and impellers areused it may be possible to use a duty cycle greater than 50%. FIG. 4shows the response of apparatus when the braking has a 33% duty cycle.

The size of the threshold is dependent on the duty cycle used to brakethe motor. With a 33% duty cycle, the threshold is set to 0.5-0.9 cmH2O, preferably 0.5 cm H2O. With 25% duty cycle, a threshold in therange of 0.2-0.7 cm H2O would be used and with 50% duty cycle athreshold in the range of 0.8-1.4 cm H2O would be used.

Since acceleration of the motor from a lower speed to a speed sufficientto provide the therapeutic pressure takes a finite time, the increase inpressure from the lower “comfort” pressure to the treatment pressurealso requires a finite time. The time delay could lead to a reducedsynchrony between patient and blower. In this form of the invention,instead of waiting until the onset of inhalation is detected (forexample, by the flow signal changing from a negative to a positivesignal), an attempt is made to predict when inhalation will occur and tocoordinate the increase in pressure to maintain synchrony.

In one form of our invention, a negative peak detector is used to allowthe pressure to start to ramp up at the apex of the exhalation cycle toallow delivery of treatment pressure to be stable before the inhalationcycle begins, as shown in FIG. 5. This method uses the previouslydescribed low threshold, with the high threshold being replaced with anegative peak detector.

In another form of our invention, we use the following method to attemptto eliminate false negative peaks. A running average of flow iscalculated from three points spaced 10 ms apart. An hysteresis thresholdmay be fitted to the moving average. This embodiment can be improved bymodifying the thresholds in the peak detection algorithm or modifyingthe approach taken to find the negative peak (for example, filtering andcalculating the differential). Negative peaks can also be detected usingthe techniques for detecting minima, such as those in “Minimization orMaximization of Functions” Chapter 10, Numerical Recipes, W. H. Press etal., 1986, Cambridge University Press, ISBN 0 521 30811 9.

Other techniques may be used to predict when inhalation will occur, tocoordinate the increase in pressure and to maintain synchrony. Forexample, in another form of the invention, an average exhalation cycleduration, t_(d), is calculated from consecutive zero-crossings of flowand the time necessary for an increase in pressure from the lower“comfort” pressure to the therapeutic pressure, t_(p). Once exhalationis detected, a timer t_(e) records the time since the start ofexhalation. Acceleration of the motor from the lower speed to the higherspeed is initiated when t_(e)=t_(d)−t_(p).

The rate of pressure decrease during free-wheeling can depend upon theinhalation pressure immediately prior to the free-wheeling onset. Forexample from an inhalation pressure of 15 cmH2O, a 3 cm H2O drop inpressure may occur in approximately 1 second. See FIG. 10. In contrastfrom an inhalation pressure of 10 cm H2O a 3 cm H2O drop in pressure mayoccur in approximately 2 seconds. See FIG. 11. Thus in accordance withan embodiment of the invention there is provided a a means for achievingan adjustable rate of exhalation pressure relief.

An advantage of the present invention in which the pressure reductionthat occurs is limited by an adjustable amount is that the limitationensures adequate CO₂ washout and/or maintenance of an open airway. Forexample, in a CPAP system in which an exhalation pressure reduction ispurely dependent on exhalation flow, the CPAP pressure during exhalationmay drop below the pressure necessary to ensure adequate CO₂ washout,which is generally about 4 cm H2O. Accordingly, an improvement can bemade to a device such as that in U.S. Pat. No. 5,535,738 or theRespironics Inc. BiPAP ventilator with C-Flex mode, wherein during anexhalation portion of the cycle, pressure is allowed to followexpiratory flow rate, but may only fall to a threshold. Thus, patientinterface pressure never falls below the pressure adequate to insuresufficient CO₂ washout and/or to maintain an open upper airway.

The freewheeling technique of this invention has been described thus faras being applied during the administration of repetitive therapy cycles.However, there are at least two other uses of the technique. One isduring the ramp phase feature available on many CPAP therapy machines,where the mask pressure builds up towards the treatment pressure duringthe ramp phase, as the patient tries to fall asleep. See U.S. Pat. Nos.5,199,424, 5,522,382 and 6,705,351. For example, during the ramp phase,the inspiratory pressure may be constant during a cycle (whileincreasing from cycle to cycle), but the expiratory pressure may beincreasing from cycle to cycle but also controlled by the motor orblower rotor freewheeling in accordance with the teachings of thepresent invention. It is also possible to disable the freewheelingtechnique at the end of the ramp phase, i.e., once the therapeuticpressure is reached. It may also be considered appropriate to taper theend of the freewheeling technique ramp phase so as to achieve a smoothtransition to the therapeutic CPAP pressure level. This tapering wouldoccur by gradually reducing the pressure differential between exhalationand inhalation over successive breaths. Preferably, this would occurduring the last minute of the ramp phase and would equate to about 15average breaths. This tapering is intended to reduce disturbance to andpromote the comfort of the patient.

By disabling the freewheeling technique at the end of the ramp phase thepatient is provided with maximum comfort while trying to fall asleep(i.e., during the ramp phase) without the freewheeling techniqueinterfering with the clinically determined therapeutic pressure duringsleep (i.e., during the therapeutic phase). Similarly, the freewheelingtechnique of the present invention can be used during the “settlingtime” phase of auto-titrating positive airway pressure (APAP) such as isavailable with a ResMed AutoSet® SPIRIT CPAP apparatus. During“settling-time” a constant but low mask pressure is provided so as toallow the patient to drift into sleep. At the end of the “settling time”the APAP provides the patient with variable therapeutic treatmentpressure as determined appropriate by the treatment algorithm. Byapplying the freewheeling technique of the present invention only during“settling time,” patient comfort may be enhanced during the transitionto sleep while not compromising the actual therapy. Of course, if it isconsidered clinically undesirable for the patient interface pressure togo below the low pressure set for “settling time” phase, then it wouldbe appropriate not to engage the freewheeling technique during thatphase. Alternatively, the settling time phase pressure may be set at alevel that would be sufficient for the freewheeling technique to beenabled but not drop the pressure during exhalation to a level that islower than a predetermined clinically acceptable level. Disabling thefreewheeling technique at the end of the ramp phase of CPAP or at theend of the “settling time” of APAP avoids the occurrence of freewheelinginduced insufficient pressure support or insufficient CO₂ washout duringthe therapeutic phase.

Although the invention has been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of the application of the principles of the invention.Numerous modifications may be made therein and other arrangements may bedevised without departing from the spirit and scope of the invention.For example, while in the illustrative embodiment, a flow signal is usedto determine the transition between inhalation and exhalation, othersignals, such as pressure, may be used. Combinations of signals, such asflow and effort, may also be used, and freewheeling may be used tocontrol or reduce patient interface pressure swing.

1. In a CPAP apparatus having an electric motor, an impeller rotated bythe motor, a patient interface, an air delivery conduit for deliveringair from the impeller to the patient interface, a sensor for determiningthe pressure in the patient interface, and a control mechanism thatcauses air to be delivered at a desired pressure to the patientinterface, and that detects transitions between inhalation andexhalation of a respiratory cycle of a patient; a method of controllingthe motor operation comprising the steps of: (i) upon detection of thetransition from inhalation to exhalation, de-energizing the motor toallow the motor to freewheel; (ii) when the pressure in the patientinterface reaches a minimum pressure level during exhalation,re-energizing the motor and controlling its speed so to maintain thepressure at a level suitable for exhalation; and (iii) upon detection ofthe transition from exhalation to inhalation, increasing the motor speedto provide higher pressures in the patient interface suitable forinhalation.
 2. The method of claim 1 wherein in step (ii) the motorspeed is controlled so as to maintain the pressure at said minimumlevel.
 3. The method of claim 1 further comprising the step of brakingthe motor during at least some of the time that it is freewheeling. 4.The method of claim 3 wherein braking of the motor occurs when the motorfirst starts to freewheel during a breathing cycle.
 5. The method ofclaim 4 wherein the step of detecting the transition between at leastone of inhalation to exhalation or exhalation to inhalation comprisesthe sub-steps of: (iv) monitoring the respiratory air flow to thepatient; and (v) comparing the air flow to a threshold.
 6. The method ofclaim 5 wherein said threshold is adjustable and when the air flow issmaller than said threshold, a transition is determined to haveoccurred, the threshold having the effect of adjusting the phase delaybetween the pressure and the air flow, the larger the threshold, thegreater the confidence that a transition has been determined reliably atthe expense of increased uncertainty in when the transition occurred,potentially leading to a phase delay and reduced synchronicity.
 7. Themethod of claim 6 wherein said minimum pressure level during exhalationis adjustable to vary the comfort level for the patient.
 8. The methodof claim 1 wherein the step of detecting the transition between at leastone of inhalation to exhalation or exhalation to inhalation comprisesthe sub-steps of: (iv) monitoring the respiratory air flow to thepatient; and (v) comparing the air flow to a threshold.
 9. The method ofclaim 8 wherein said threshold is adjustable and when the air flow issmaller than said threshold, a transition is determined to haveoccurred, the threshold having the effect of adjusting the phase delaybetween the pressure and the air flow, the larger the threshold, thegreater the confidence that a transition has been determined reliably atthe expense of increased uncertainty in when the transition occurred,potentially leading to a phase delay and reduced synchronicity.
 10. Themethod of claim 9 wherein said minimum pressure level during exhalationis adjustable to vary the comfort level for the patient.
 11. The methodof claim 1 wherein said minimum pressure level during exhalation isadjustable to vary the comfort level for the patient.
 12. The method ofclaim 1 further comprising the step of braking the motor in a ditheredfashion during at least some of the time that it is freewheeling. 13.The method of claim 12 wherein braking of the motor occurs when themotor first starts to freewheel during a breathing cycle.
 14. A methodof operating a CPAP apparatus in which a motor-driven rotating impellerdelivers air at a desired pressure to a patient interface, comprisingthe steps of: (i) detecting the transition from inhalation to exhalationand in response thereto allowing the impeller to freewheel; (ii)detecting when the pressure in the patient interface reaches a minimumpressure level during exhalation and in response thereto causing themotor to control the impeller speed so as to maintain the pressure at alevel suitable for exhalation; and (iii) detecting the transition fromexhalation to inhalation and in response thereto causing the motor tocontrol the impeller to operate at a higher speed to provide higherpressures in the patient interface suitable for inhalation.
 15. Themethod of claim 14 wherein in step (ii) the impeller speed is controlledso as to maintain the pressure at said minimum level.
 16. The method ofclaim 14 wherein the impeller is connected to a motor and furthercomprising the step of braking the motor during at least some of thetime that the impeller is freewheeling.
 17. The method of claim 16wherein braking of the motor occurs when the impeller first starts tofreewheel during a breathing cycle.
 18. The method of claim 14 whereindetecting the transition between at least one of inhalation toexhalation or exhalation to inhalation comprises the sub-steps of: (iv)monitoring the respiratory air flow to the patient; and (v) comparingthe air flow to a threshold.
 19. The method of claim 18 wherein saidthreshold is adjustable and a transition is detected when the air flowis smaller than said threshold, the threshold having the effect ofadjusting the phase delay between the pressure and the air flow, thelarger the threshold, the greater the confidence that a transition hasbeen detected reliably at the expense of increased uncertainty in whenthe transition occurred, potentially leading to a phase delay andreduced synchronicity.
 20. The method of claim 14 wherein said minimumpressure level during exhalation is adjustable to vary the comfort levelfor the patient.
 21. The method of claim 14 wherein the impeller isconnected to a motor and further comprising the step of braking themotor in a dithered fashion during at least some of the time that theimpeller is freewheeling.
 22. A CPAP apparatus for treatment of sleepdisordered breathing by having a blower driven by a motor provide air ata desired pressure at the airway of a patient, in which the pressureduring exhalation is reduced from a high level to a low level byallowing the motor to freewheel.
 23. A CPAP apparatus for treatment ofsleep disordered breathing by having an impeller driven by a motorprovide air at a desired pressure at the airway of a patient, in whichthe pressure during exhalation is reduced from a high level to a lowlevel by allowing the impeller to freewheel.
 24. A CPAP apparatus fortreatment of sleep disordered breathing by having a blower driven by amotor provide air at a desired pressure at the airway of a patient, inwhich the pressure during exhalation is reduced from a high level to alow level by allowing the motor to freewheel only during a pretreatmentphase of use of the CPAP apparatus.
 25. A CPAP apparatus of claim 24where said pretreatment phase of use is a ramp phase.
 26. A CPAPapparatus of claim 24 where said pretreatment phase of use is settlingtime.
 27. A CPAP apparatus having two modes of operation: a) a firstmode where the pressure during exhalation is reduced from a high levelas claimed in any one of claims 1, 22 or 23, and b) a second mode wherethe pressure remains substantially constant during a cycle of inhalationand exhalation; and a controller adapted to detect the presence of anapnea and to switch the CPAP apparatus from the first mode of operationto the second mode of operation upon detection of an apnea.
 28. A CPAPapparatus of claim 27 where the controller is further adapted to detectthe resumption of normal breathing and to switch from the second mode ofoperation to the first mode of operation upon the detection of theresumption of normal breathing.
 29. A CPAP apparatus of claim 27 wherethe controller is further adapted to switch from the second mode ofoperation to the first mode of operation upon the expiration of apredetermined period of time.
 30. A method of operating a CPAP apparatushaving two modes of operation: a) a first mode where the pressure duringexhalation is reduced from a high level as claimed in claim 14, and b) asecond mode where the pressure remains substantially constant during acycle of inhalation and exhalation; and a controller adapted to detectthe presence on an apnea and to switch the CPAP apparatus from the firstmode of operation to the second mode of operation upon detection of anapnea.
 31. A method of operating a CPAP apparatus as claimed in claim 30where the controller is further adapted to detect the resumption ofnormal breathing and to switch from the second mode of operation to thefirst mode of operation upon the detection of the resumption of normalbreathing.
 32. A method of operating a CPAP apparatus as claimed inclaim 30 where the controller is further adapted to switch from thesecond mode of operation to the first mode of operation upon theexpiration of a predetermined period of time.